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The ID prefix is shared between the Wayland client code in Lacros and
the Wayland server code in ash, so share the constant.

Bug: none
Change-Id: If62550c447670e553a854ce1092cfd3d7d9aacff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427546Reviewed-by: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810277}